Musnad Abu Ya'la al-Mawsili 49:4

Abdullah ibn Umar ibn Aban al-Kufi, the nephew of Husayn al-Ju'fi, narrated to us, saying: Abd al-Aziz ibn Muhammad al-Darawardi narrated to us, from Zayd ibn Aslam, from a son of Abu Waqid al-Laythi, the companion of the Messenger of Allah (may Allah bless him and grant him peace), from his father, that he said: I heard the Messenger of Allah (may Allah bless him and grant him peace) saying to his wives in the year of the Farewell Pilgrimage: "This, then the backs of the mats."